This invention relates to information signal processing in general, and in particular to the field of processing time sequential information signals (such as video signals) for the purpose of compressing the amount of information to be transferred from an encoding site to a decoding site.
In recent years, increasing efforts have been directed toward providing more efficient information signal encoding techniques used to process time sequential information signals prior to their transmission from a transmitting station to a receiving station. The requirement for more efficient encoding techniques has been prompted by two major factors: firstly, a substantial increase in the quantity of information required to be transferred via communication links and, secondly, maximum occupancy of the communication frequency bands available for voice and data transmission. An early technique employed to reduce the amount of information required to be transferred without substantial degradation is the signal processing technique known as conditional replenishment, described in U.S. Patent No. 3,984,626 to Mounts et al., the disclosure of which is hereby incorporated by reference. Briefly, in the conditional replenishment signal processing technique, individual line element sample signals from a successive field of information are compared with the corresponding line elements in the previous field, and the difference therebetween is tested against a fixed threshold. If the difference exceeds the threshold value, the new value is encoded and transmitted to a receiving station, along with an appropriate address code specifying the line location of the sample to be updated in the field memory of the receiving station. Thus, rather than transmitting each and every line sample for every field, only those samples which differ by a significant threshold amount are transmitted, which substantially reduces the number of samples in the communication channel pipeline. Although this saving in the amount of actual data flowing through the communication pipeline is somewhat offset by the necessity of simultaneously transmitting the address information, this disadvantage is more than overcome by the substantial reduction in the total number of samples which must be transmitted in order to maintain the information current at the decoding site. When used to process video type information signals, an even greater reduction in the required number of transmitted samples is achieved due to the inherent nature of video signals, which possess intrinsic interfield correlation (e.g. abrupt interfield changes for background portions of video images occur relatively infrequently).
Another compression technique known in the art is the use of transform domain encoding, in which each field of information signals is divided into a number of rectangular or square arrays of individual picture elements (for example a 16 pixel by 16 pixel array) termed blocks, and each block is converted to the transform domain. For each converted block, the individual transform coefficients are then encoded and transmitted along with appropriate address codes, as well as additional overhead information (e.g. field start signals, frame start signals and the like). One such transform domain processing system is disclosed in U.S. Pat. No. 4,189,748 to Reis, the disclosure of which is hereby incorporated by reference.
Although many types of mathematical transform functions have been proposed for implementation in a transform domain signal processing system, in reality most transform functions are inappropriate for implementation due to the complexity of the required logic circuitry. This disadvantage is exacerbated in applications requiring real time signal processing by virtue of the minimum time period required to perform the signal processing necessary to generate the values of the transform coefficients. For a general discussion of the advantages and disadvantages of the different types of transform functions, reference should be had to the collection of technical publications entitled "Image Transmission Techniques, Advances in Electronics and Electron Physics, Supplement 12", Pratt, Academic Press, 1979, particularly the section entitled "Transform Image Coding".